The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus which is capable of stably and efficiently conveying developer.
The “background” description provided herein is for the purpose of generally presenting the context of the invention. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present inventions.
Background image forming apparatuses, such as printers, facsimiles, copiers, and multifunction apparatuses which print, fax, copy, and so on generally use an electrophotographic process for image forming. Such background image forming apparatuses include a development unit which develops a latent image formed on an image carrier, such as a photosensitive drum. In the development unit, developer is conveyed to develop the image.
The development unit generally includes a screw member having a spiral shape to convey developer. The developer is conveyed along the axis of the screw member by rotating around a shaft center of the screw member.
A background development unit includes first and second agitating screws and a development roller. The first and second agitating screws are arranged in a developer storage and include projected spiral vanes around a shaft member of the agitating screw to agitate and convey the developer. The development roller is arranged in a development unit located next to the developer storage. The development roller conveys the latent image to a position the image carrier faces by the rotation of the development roller and develops the latent image with the developer.
A partition plate may or may not be arranged between the developer storage and the development unit. The development roller in the development unit and the second agitating screw in the developer storage are arranged to face each other with each circumference keeping a parallel positional relationship with a predetermined distance.
In the developer storage, first and second storage compartments are separately formed with a partition wall. The second agitating screw is arranged in the second storage compartment located closer to the development unit. The first agitating screw is arranged in the first storage compartment located on the far side of the development unit.
The developer in the first storage compartment is conveyed in a shaft axis direction of the first agitating screw by rotating around a shaft center of the first agitating screw. The developer is conveyed to an end of the first agitating screw. The developer is transferred into the second storage compartment from the first storage compartment through a communication hole.
Developer is conveyed in the second storage compartment by the rotation of the second agitating screw, part of the developer is pumped up by the development roller to contribute to the development and is returned to the second agitating screw from the development roller again. When the developer is conveyed to an end of the second agitating screw, the developer is transferred into the first storage compartment from the second storage compartment through a communication hole.
In this configuration, an unevenness of development concentration may occur because the surface of the developer being conveyed by the second agitating screw is undulated. More specifically, in a cross section of the second agitating screw, the shaft member has a projection of spiral vanes from part of an outer border of the rounded shaft member. The projection is rotated around the shaft member in accordance with the rotation of the shaft member. While the projection is moving from a farthest position to a closest position with respect to the development roller, the developer is pushed to the development roller and the surface of the developer is lifted up gradually in a direction to the development roller. The amount of developer in the region between the second agitating screw and the development roller is increased and the developer concentration is also increased.
Meanwhile, while the projection is moving from the closest position to the farthest position with respect to the development roller, the surface of the developer falls down gradually because the developer is moved in the same direction as the movement of the projection. The amount of developer in the region between the second agitating screw and the development roller is decreased and the developer concentration is also decreased.
The lifting-up and falling-down of the developer surface occurs during the rotation phase because the projection of the spiral vane is formed to have a spiral shape in a direction of the shaft member. As a result, an unevenness of development concentration may occur in accordance with the surface undulation of the developer in the direction of the screw axis.
Another background development unit includes a multiple thread screw having a plurality of spiral vanes, for example, a two spiral vane screw structure. Referring to a cross section of the two spiral vane screw, a first spiral vane projects from part of an outer border of the rounded shaft member. A second spiral vane projects at an opposite portion symmetrically, 180 degrees from the first spiral vane.
With respect to the two spiral vane screw, while one projection is moving from a farthest position to a closer position with respect to the development roller, the other projection is moving further from the development roller. The lifting-up of the developer is canceled due to the effect of a decrease of the developer surface, caused by the movement of the other spiral vane. As a result, an unevenness of development concentration due to the surface undulation of the developer is avoided.
However, the agitation of the developer during the conveyance may be weaker because the movement of the developer is limited within a region between the first and second vanes when using the two spiral vane screw. The agitation efficiency of the two spiral vane screw may be worse than the agitation efficiency of the single spiral vane screw. Therefore, some problems may occur due to the decrease of the agitation efficiency.
For example, when a two-component developer which includes toner and magnetic carrier is used, unevenness of the toner of the two-component developer occurs due to insufficient agitation and unevenness of the development concentration. When another two-component developer which includes toner and non magnetic carrier is used, the agitation of fully charged toner with partly charged toner may be insufficient. Thus, the partly charged toner may accumulate at a particular position and may cause a scumming phenomenon in which the partly charged toner is attached at non image area.